Inverter selection for rooftop solar system
Inverters are electrical devices that convert the DC power generated by solar PV modules into AC power. They consist of thyristors or insulated gate bipolar transistors (IGBTs), which alternate the polarity of the DC current by means of pulse width modulation (PWM) to create a sinusoidal voltage output to match the local grid code. The inverters have the following key functions:
» Convert DC power to AC power, allowing power evacuation to the grid.
» Maximum power point tracking (MPPT) to operate the inverter at the best operating point: the maximum power point (MPP) in the current-voltage (IV) curve depends on solar irradiation (see Figure 17).
» Galvanic isolation: an internal transformer provides isolation from the grid to meet safety requirements.
» Anti-islanding: grid-tied inverters are equipped with anti-islanding circuitry. This is necessary because, in case of a blackout, solar PV modules will continue feeding electricity into the grid as long as there is irradiation.
» Monitoring: usually the inverters have their own monitoring tool with remote Internet access.
» Contribution to grid stability: by providing reactive power.